The appearance of atherosclerotic lesions at specific locations in the arterial tree has led many investigators to study the relevance of hemodynamic factors in atherogenesis. The purpose of this study is to elucidate the role of fuid dynamics and mass transfer in the onset and development of atherosclerotic plaques. Specifically, the wall shear stress was measured in a plastic model of a canine artery using an electrochemical technique to determine velocity gradients at the fluid-wall interface. Regions of high shear and of disturbed flow patterns in the model have been shown to correlate with regions of high incidence of lesions in animals on an atherogenic diet. These regions were near branches and flow dividers. The mechanism by which various flow patterns mediate increased lipid uptake by the arterial wall must still be determined.